As the world's human population expands, greater amounts of energy are necessary to provide the economic growth all nations desire. The traditional sources of energy are the fossil fuels which, when consumed, create significant amounts of carbon dioxide as well as other more immediately toxic materials including carbon monoxide, sulfur oxides, and nitrogen oxides. Increasing atmospheric concentrations of carbon dioxide are warming the earth; bringing about global climatic changes. Energy-producing devices which do not contribute to such difficulties are needed now.
A fuel cell is an energy-conversion device that directly converts the energy of a supplied gas into an electric energy. Highly efficient fuel cells employing hydrogen, particularly with their simple combustion product of water, would seem an ideal alternative to current typical power generations means. Researchers have been actively studying such devices to utilize the fuel cell's potential high energy-generation efficiency.
Fuel cells, like batteries, operate by utilizing electrochemical reactions. Unlike a battery, in which chemical energy is stored within the cell, fuel cells generally are supplied with reactants from outside the cell. Barring failure of the electrodes, as long as the fuel, preferably hydrogen, and oxidant, typically air or oxygen, are supplied and the reaction products are removed, the cell continues to operate.
Fuel cells offer a number of important advantages over internal combustion engine or generator systems. These include relatively high efficiency, environmentally clean operation especially when utilizing hydrogen as a fuel, high reliability, few moving parts, and quiet operation. Fuel cells potentially are more efficient than other conventional power sources based upon the Carnot cycle.
The major components of a typical fuel cell are the hydrogen electrode for hydrogen oxidation and the oxygen electrode for oxygen reduction, both being positioned in a cell containing an electrolyte (such as an alkaline electrolytic solution). Typically, the reactants, such as hydrogen and oxygen, are respectively fed through a porous hydrogen electrode and a porous oxygen electrode and brought into surface contact with the electrolytic solution. The particular materials utilized for the oxygen electrode and hydrogen electrode are important since they must act as efficient catalysts for the reactions taking place.
In an alkaline fuel cell, the reaction at the hydrogen electrode occurs between the hydrogen fuel and hydroxyl ions (OH−) present in the electrolyte, which react to form water and release electrons:H2+2OH−→2H2O+2e− (E0=−0.828 v).At the oxygen electrode, the oxygen, water, and electrons react in the presence of the oxygen electrode catalyst to reduce the oxygen and form hydroxyl ions (OH−):O2+2H2O+4e−→4OH− (E0=−0.401 v).The total reaction, therefore, is:2H2+O2→2H2O (E0=−1.229 v).The flow of electrons is utilized to provide electrical energy for a load externally connected to the hydrogen electrode and oxygen electrode.
In order to achieve high current densities, most of the fuel cells operating at ambient or near ambient conditions utilize high surface area electrodes. In such electrodes, carbon plays a very important role. Carbon has as least one or more orders of magnitude higher surface area compared to the other traditionally used materials. Carbon is also relatively inexpensive, chemically stable, and inert to most electrolytes. Generally, by varying the starting materials and the processing conditions, it has been possible to produce a variety of carbons with varying surface areas.
Fuel cell electrodes have a gas diffusion layer that is permeable to gases such as hydrogen and oxygen and an active layer which is in contact with the electrolyte. In the hydrogen electrode, hydrogen gas passes through the gas diffusion layer and is uniformly distributed to the active layer of the hydrogen electrode. The active layer dissociates the hydrogen, absorbs the hydrogen and reacts the hydrogen at the electrolyte interface. In the oxygen electrode, oxygen enters the gas diffusion layer and is adsorbed. The adsorbed oxygen is then distributed to the active layer where the oxygen is reacted at the electrolyte interface.
One of the biggest problems facing fuel cell development is the tendency of the electrodes to “flood” over the life time of the fuel cell. In other words, electrolyte penetration into the bulk of the electrode leads to a loss of active catalyst area, usually resulting in a poor performance of the fuel cell. With this in mind, the gas diffusion layer is designed to be highly hydrophobic. Since the gas diffusion layers are not directly involved in the current collection and cannot tolerate any flooding, they are designed to have higher hydrophobic content as compared to the other layers of the electrodes. The active layer (AL) where electrochemical reactions take place and where lower resistance is desired, the electrode is made less hydrophobic. Hydrophobicity is introduced by coating the carbon in the gas diffusion layer with polytetrafluoroethylene.
The gas diffusion layer is a porous matrix that distributes the hydrogen or oxygen in a uniform manner across the entire face of the respective electrode surfaces. Porosity and pore structure are important in this layer whereas surface area is of only secondary importance unlike the active layer. Typically, the oxygen electrochemical reduction kinetics is very slow depending strongly on the applied current density. To minimize the polarization, the oxygen electrode is operated at low current densities. Operation at low current densities (with reasonably good power output) is achieved by increasing the total surface area while keeping the same geometrical area. So the carbon particles used for the active layer usually have higher surface areas.
The process by which the carbon particles are coated with polytetrafluoroethylene results in partial covering of the carbon particles. Subsequently when they are compacted in a hydraulic press or in a roll mill (sometimes with a post sintering operation) the polytetrafluoroethylene gets stretched and forms a network holding the carbon particles in its matrix. During the operation in a full cell mode over a long operational time, the electrodes gradually begin to “flood” and the performance goes down. The hot electrolyte may gradually undermine the polytetrafluoroethylene coating thus rendering the electrode vulnerable to flooding. Increasing the polytetrafluoroethylene content to prevent “flooding” is not an answer as it tends to “gum up” and the pores become closed. On the active layer side, increased polytetrafluoroethylene increases the resistance within the electrode. Use of a material in the gas diffusion layer with built in hydrophobic properties would enable the gas diffusion layer to remain hydrophobic even when the polytetrafluoroethylene is undermined by the electrolyte solution. The use of a material in the gas diffusion layer with built in hydrophobic properties would also allow the amount of polytetrafluoroethylene in the gas diffusion layer to be reduced thereby increasing the porosity of the gas diffusion layer.
The present invention discloses electrodes incorporating a fluorinated carbon. The fluorinated carbon has a built in hydrophobic property which reduces the amount of polytetrafluoroethylene in the electrode. The fluorinated carbon enables the gas diffusion layer of the electrode to remain hydrophobic even after the polytetrafluoroethylene has begun to wear away. Electrodes produced using the fluorinated carbon exhibit excellent hydrophobic characteristics where needed in the electrodes and have a life time longer than those currently available.